Corrugated Stainless Steel Tubing (CSST) is a relatively new building product used to plumb structures for fuel gas in lieu of conventional black pipe. The advantages that are offered for CSST include a lack of connection and a lack of threading. In essence, it is a material that results in substantial labor savings relative to using black pipe.
The use of Corrugated Stainless Steel Tubing (CSST) to serve as a conduit for delivering fuel gas within residential and commercial buildings has been recognized by the National Fuel Gas Code (NFPA 54) since about 1988. Various code bodies and regulatory agencies have allowed the use of CSST in such structures.
CSST differs from black pipe in a number of ways. In a CSST system, gas enters a house at a pressure of about 2 psi and is dropped to ˜7″ WC by a regulator in the attic (assuming a natural gas system). The gas then enters a manifold and is distributed to each separate appliance via “home runs.” Unlike black pipe, a CSST system requires a separate run for each appliance. For example, a large furnace and two water heaters in a utility closet will require three separate CSST runs. With black pipe, the plumber may use only one run of 1″ pipe and then tee off in the utility room. Therefore, the requirement of one home run per appliance significantly increases the number of feet of piping in a building.
CSST is sold in spools of hundreds of feet and is cut to length in the field for each run. In this regard, CSST has no splices or joints behind walls that might fail. CSST also offers an advantage over black pipe in terms of structural shift. With black pipe systems, the accommodations for vibrations and/or structural shifts are handled by appliance connectors, a form of flexible piping.
Unfortunately, a major drawback to the use of CSST is the propensity for it to fail when exposed to an electrical insult such as from a lightning strike to an adjacent structure. CSST is very thin, with walls typically about 10 mils in thickness. The desire for easy routing of the tubing necessitates this lack of mass. However, it also results in a material through which electricity can easily puncture.
When subjected to significant electrical insult such as a lightning strike, CSST typically develops holes which act as orifices for raw fuel gas leakage. Even worse, the electrical arcing process which causes the insult and resultant gas leak from the CSST will often ignite the gas, effectively turning the gas leak into a blowtorch. This phenomenon is described by the inventor's two papers on the subject, “CSST and Lightning,” Proceedings, Fire and Materials 2005 Conference, January 2005, and “The Link Between Lightning, CSST, and Fires,” Fire and Arson Investigator, October 2005, the contents of which are hereby incorporated by reference.
Lightning strikes vary in current from 1,000 (low end) to 10,000 (typical) to 200,000 (maximum) amperes peak. Mechanical damage caused by heating is a function of the current squared multiplied by time. Thus, the current is the dominant factor creating the melting of gas tubing.
One of the underlying issues with CSST is that it is part of the electrical grounding system. For reasons of electric shock prevention (and also elimination of sparks associated with static electricity), it is desirable to have all exposed metal within a structure bonded so that there are no differences of potential. However, there are limitations to applying DC circuit theory (or even 60 Hz steady state phasor theory) in this situation because lightning is known to have fast wavefronts. While the reaction of large wires and irregular surfaces is predictable at 60 Hz, the fast wave fronts associated with lightning may cause substantial problems with CSST, given its corrugated surface. Moreover, new house construction has shown very tight bends and routing of CSST immediately adjacent to large ground surfaces, creating the potential for arcs created by lightning strikes. Testing of CSST under actual installed conditions using transient waveforms may well show further limitations that conventional bonding and grounding cannot accommodate.
The typical gas line or gas system, whether black pipe or CSST, is usually not a good ground. The metal components that make up a gas train are made from materials that are chosen for their ability to safely carry natural gas (or propane) and the accompanying odorant. These metallic components are not known for their ability to carry electric current. To further compound matters, it is not uncommon to find pipe joints treated with Teflon tape or plumber's putty, neither of which is considered an electrical conductor. The Fuel Gas Code (NFPA 54) calls for above ground gas piping systems to be electrically continuous and bonded to the grounding system. The code provision also prohibits the use of gas piping as the grounding conductor or electrode.
Gas appliance connectors (GAC), which are prefabricated corrugated gas pipes, are also known to fail from electric current, whether this current is from lightning or from fault currents seeking a ground return path. These connectors usually fail by melting at their ends (flares) during times of electrical overstress. These appliance connectors are better described ANSI Z21.24, Connectors for Indoor Gas Appliances, the contents of which are hereby incorporated by reference. A gas appliance that is not properly grounded is more susceptible to gas line arcing than a properly grounded appliance. The exact amount of fault current, however, will depend upon the impedances of the several ground paths and the total fault current that is available. For example, air handlers for old gas furnaces seem to be the most prone. Typically, an inspection will reveal that the power for the blower motor uses a two-conductor (i.e. non-grounded) power cord.
A primary indicator that is found in these types of fires is the focal melting of the gas line at the brass nut/connector. It is well known and appreciated that the flame that is fueled from a gas orifice does not normally make physical contact with the orifice itself. Rather, there is some distance between the flame and orifice depending on the gas pressure, the size of the orifice, available oxygen, and the mixing or turbulence. In short, the leaking gas is too rich to bum at the point of escape. In addition, gas that is under pressure will cause a very small amount of cooling to occur when the gas escapes from such a leak or orifice due to adiabatic cooling. Both of these factors indicate that a gas line would be least likely to melt at a connection if the melting were indeed caused by the heat from a flame, as opposed to electrical insult.
Therefore, it would be desirable to have a gas conduit system incorporating CSST or GAC that is capable of preventing fires caused by auto-ignition of gas leaks resulting from electrical insult to the gas tubing.